For many processors, executing a move instruction usually involves moving a value from one register to another register, or moving an immediate to a register. Because of the frequency of such move instructions, processor performance may be increased if move instructions are efficiently processed.
Most instructions operate on several source operands and generate results. They name, either explicitly or through an indirection, the source and destination locations where values are read from or written to. A name may be either a logical (architectural) register or a location in memory.
Usually, the number of physical registers available in a processor exceeds the number of logical registers, so that register renaming may be utilized to increase performance. In particular, for out-of-order processors, register renaming allows instructions to be executed out of their original program order. Thus, for many out-of-order processors, a move instruction is renamed so that logical registers named in the original move instruction are renamed to physical registers.
Renaming a logical register involves mapping a logical register to a physical register. These mappings are stored in a RAT (register alias table or register allocation table). A RAT maintains the latest mapping for each logical register. A RAT is indexed by logical registers, and provides mappings to corresponding physical registers.